MIT Kerberos 5 ASN.1 decoding functions insecurely deallocate memory (double-free)

ID VU:795632
Type cert
Reporter CERT
Modified 2005-05-10T00:00:00



The MIT Kerberos 5 library does not securely deallocate heap memory when decoding ASN.1 structures, resulting in double-free vulnerabilities. An unauthenticated, remote attacker could execute arbitrary code on a KDC server, which could compromise an entire Kerberos realm. An attacker may also be able to execute arbitrary code on Kerberos clients, or cause a denial of service on KDCs or clients.


As described on the MIT Kerberos web site: "Kerberos is a network authentication protocol. It is designed to provide strong authentication for client/server applications by using secret-key cryptography." MIT Kerberos code is used in network applications from a variety of different vendors and is included in many UNIX and Linux distributions.

Kerberos 5 protocol messages are defined using Abstract Syntax Notation One (ASN.1). When ASN.1 decoding functions in the MIT Kerberos 5 library handle error conditions, the functions free() a memory reference and return the reference to the calling function. In some cases, error handling code in the calling functions may free() the memory reference again, resulting in a double-free vulnerability. MITKRB5-SA-2004-002 explains in more detail:

In the MIT krb5 library, in all releases up to and including krb5-1.3.4, ASN.1 decoder functions and their callers do not use a consistent set of memory management conventions. The callers expect the decoders to allocate memory. The callers typically have error-handling code which frees memory allocated by the ASN.1 decoders if pointers to the allocated memory are non-null. Upon encountering error conditions, the ASN.1 decoders themselves free memory which they have allocated, but do not null the corresponding pointers. When some library functions receive errors from the ASN.1 decoders, they attempt to pass the non-null pointer (which points to freed memory) to free(),
causing a double-free.
The MIT Kerberos 5 KDC is affected by a specific variant of this type of double-free condition. From MITKRB5-SA-2004-002:

In all releases of MIT krb5 up to and including krb5-1.3.4, cleanup code in the KDC frees memory returned by ASN.1 decoders. This cleanup code only frees memory pointed to by non-null pointers, but if an ASN.1 decoder returns an error, the cleanup code will free memory previously freed by the decoder.
The double-free conditions occur in the MIT Kerberos 5 library and affect the KDC and Kerberos clients.


An unauthenticated, remote attacker could execute arbitrary code on a KDC server. This could allow an attacker to gain the master secret for a Kerberos realm, leading to compromise of the entire realm. An attacker who is able to impersonate a KDC or application server may be able to execute arbitrary code on Kerberos clients. An attacker may also be able to crash a KDC or client, causing a denial of service.


Apply a patch
Apply the appropriate patch(es) referenced in MITKRB5-SA-2004-002 or specified by your vendor.


According to MITKRB5-SA-2004-002, "The upcoming krb5-1.3.5 release will contain fixes for these problems."

Restrict access

Depending on network architecture, it may be practical to restrict access to KDC servers (88/udp) from untrusted networks such as the Internet. Due to network application requirements, it may be possible, but less practical, to limit access from Kerberos clients to trusted KDC and application servers. While these workarounds will help to limit the source of attacks, they will not prevent attacks from trusted hosts or networks or attackers who can successfully spoof their source addresses.

Systems Affected

Vendor| Status| Date Notified| Date Updated
Apple Computer Inc.| | 21 Jul 2004| 10 May 2005
Debian| | 21 Jul 2004| 03 Sep 2004
Fedora Legacy Project| | -| 03 Sep 2004
MandrakeSoft| | 21 Jul 2004| 03 Sep 2004
MIT Kerberos Development Team| | -| 01 Sep 2004
Red Hat Inc.| | 21 Jul 2004| 02 Sep 2004
Trustix Secure Linux| | -| 03 Sep 2004
Cisco Systems Inc.| | 21 Jul 2004| 02 Sep 2004
CyberSafe| | -| 02 Sep 2004
Hitachi| | 21 Jul 2004| 03 Sep 2004
VanDyke Software Inc.| | 21 Jul 2004| 02 Sep 2004
WRQ| | 21 Jul 2004| 02 Sep 2004
Conectiva| | 21 Jul 2004| 02 Sep 2004
Cray Inc.| | 21 Jul 2004| 02 Sep 2004
EMC Corporation| | 21 Jul 2004| 02 Sep 2004
If you are a vendor and your product is affected, let us know.

CVSS Metrics

Group | Score | Vector
Base | N/A | N/A
Temporal | N/A | N/A
Environmental | N/A | N/A


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Thanks to Tom Yu and the MIT Kerberos Development Team for reporting this vulnerability and coordinating with vendors. MITKRB5-SA-2004-002 acknowledges Will Fiveash and Nico Williams.

This document was written by Art Manion.

Other Information

  • CVE IDs: CAN-2004-0642
  • Date Public: 31 Aug 2004
  • Date First Published: 01 Sep 2004
  • Date Last Updated: 10 May 2005
  • Severity Metric: 20.55
  • Document Revision: 42